The present invention relates generally to surfacing optical lenses having at least one surface of revolution and more particularly though not exclusively to such optical lenses having two spherical faces and having a relatively small ratio of thickness to diameter.
Such is the case for example with ophthalmic lenses namely afocal ophthalmic lenses such as those of tinted glass adapted for use in sunglasses. Such afocal ophthalmic lenses usually having a relatively small thickness for a relatively larger diameter.
The front and rear faces of such afocal ophthalmic lenses are normally rigorously concentric at all points such that a light ray entering through any point on the front face exits through a substantially axially aligned point without any deviation other than that due to mere transverse offset caused -y the refraction of the thickness of the material traversed.
Although the front and rear faces of such an afocal ophthalmic lens are commonly referred to as parallel, such an afocal ophthalmic lens may be considered a plate with parallel faces at all points.
Usually the surfacing of such an optical lens having at least one spherical surface is carried out by securement in a workpiece holder and the workpiece holder applying the lens against a rotatable surfacing tool.
To the present day the optical lens to be surfaced is typically secured rigidly to such a workpiece holder either by a spot of low melting point metal or by suction.
Now, with afocal ophthalmic lenses it is not uncommon to find, if only locally, a defect of parallelism between the front and rear faces of such an optical lens referred to as a prism effect, a light ray being deviated with respect to its normal optical path through the optical lens as if it traversed a prism.
The reason may reside notably in the fact that since the optical lens is relatively rigidly secured to the workpiece holder which applies it against the surfacing tool, the optical lens has no freedom of movement relative to the workpiece holder and may therefore be locally squeezed or pinched between the workpiece holder and the surfacing tool, with nonuniform removal of material along the surface, if its geometrical axis does not coincide with that of workpiece holder.
Now, in practice, there is rather frequently a deviation between the geometrical axis of such a lens to be surfaced and the axis either of the spot of low melting point metal or the recess in which the lens is accommodated when it is held by negative pressure, and therefore that of the corresponding workpiece holder.
If the deviation between the axes does not exceed 0.01 to 0.02 mm, the surface of the lens concerned may be considered to be uniformly surfaced. Such is not the case when the deviation is greater. Yet with the formation of a spot of low melting point metal on the lens to be surfaced or with the recess for receiving the lens when hold by negative pressure, mounting tolerances are generally greater than 0.1 mm for the distances between the axes. Moreover, in addition to the effect of such deviation between the axes is that due to the want of alignment between the geometrical axis of the lens to be surfaced and the axis of its peripheral edge. Such deviation and/or want of alignment of the axes is the source of the prism effect to be avoided.
In German DAS 1,041,832 which pertains to surfacing of lens workpieces whose ratio of thickness to diameter is relatively large, such as in piezo-electric elements or certain optical lenses for optical instruments in particular, in which there is a chamber for receiving the lens connected by a conduit to a source of compressed air. In an embodiment of this patent there is a pneumatic cushion formed between the endwall of the cavity and the workpiece, with radial play illustrated in the drawing between the peripheral edge of the workpiece and the sidewall of the chamber. A return line carrying the compressed air from the chamber back toward the source.
Such an arrangement is not suitable for surfacing relatively thin workpiece and when there is no control of the position of the workpiece relative to the chamber in which it is received.